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If you carefully inspect the wiring within the motor you should find that the current passes from one terminal through each of the field windings and the rotor (via the commutator) all connected in series. Using a multimeter (How to use a multimeter) check the resistance of each of the field windings and of the rotor. If either of the field windings shows as an open circuit then that is the fault. It could be a fractured wire or the winding could have burned out (in which case there are likely to be signs of overheating). In the case of a fractured wire the fracture may well be hidden, and even if not it may be very difficult to access or repair.